Q: I’m currently dealing with insulating a cathedral ceiling, which has become a major issue. I’ve been researching solutions but feel stuck in figuring out what’s the best way to go about dealing with the problem.
I have a shed roof with 2:12 pitch, built with 2×8 roof joists. It’s a mobile home that’s 14×76, so the roof is just one continuous slope. I live in northern Oregon with lots of rain in my area and occasional freezes.
The issues began when I discovered that my newly installed asphalt shingles never got sealed by sun exposure, creating a massive roof leak. I ended up gluing them with Henry’s Wet Patch adhesive, which seems to have solved that problem.
The second issue is that I’m one of those people who was after the highest R-value in the ceiling. So I used R-30 faced fiberglass insulation which didn’t leave any vent space beneath the roof sheathing. I pulled the insulation out and used plastic vent channels beneath the sheathing, then stuffed R-30 back in there, compressing it a little.
A few days later I checked, and saw that there was lots of moisture accumulation on the bottom of those plastic spacers. I’m talking about visible large droplets along the entire bottom surface of the spacers!
I want to mention that I do have vent holes in both bottom and top eaves. After reading your comments on a previous cathedral ceiling question, I’m wondering if I should thin out the R-30 batts to where it is closer to R-15, take those plastic spacers out, and then screw foil-faced foam board beneath the roof joists.
Or should I just take the fiberglass out and use only foam boards. Another thing I’m considering is whether the moisture that I’m seeing is from roof sheathing drying after that roof leak…
I’m about to re-do the ceiling for the third time and really want It to be finally done right. Please advise if you can. Thank you – I appreciate your time! — Ilya
A: Let’s start at the top. Most asphalt roofing manufacturers recommend a minimum slope of 4:12. For roofs between 2:12 and 4:12, they typically recommend double-coverage underlayment (that is, lapped 19 in. for a 36 in. roll) or peel-and-stick membrane, which would be my preference, especially in a wet climate like yours. So the water you are seeing on your plastic spacers could be from roof leakage.
Most likely, however, the water droplets are condensation of moisture from inside the house. Warm interior air is always moving upward in cold weather due to the stack effect. The plastic vent baffles are the first cold surface reached by moist interior air that leaks into the roof structure in cold weather.
So the plastic vent baffles become the “plane of condensation”. Whenever a cold surface is below the dew point of the surrounding air, condensation will occur. Think cold beer can on a hot, humid day.
In a small house like yours, the indoor humidity is often high when the house is closed up in cold weather. Spot ventilation can help, especially in the kitchen and bathrooms. If you have condensation running down the inside face of double-pane windows, that’s an indication that indoor humidity is too high. If spot ventilation is not enough, whole-house ventilation may be needed. A timer on a bath fan is the simplest, cheapest approach.
Airtight Air Barrier
The other key to keeping moist air out of your roof structure is an airtight air barrier at the ceiling level, below the insulated rafter cavities. This can be achieved in a number of ways. The most common are
- Foamboard that is carefully taped at the seams and sealed around the perimeter with canned foam
- Drywall that is carefully sealed to the framing with high-quality gaskets and sealants at all junctions wit exterior walls and interior partitions
- A continuous vapor retarder such as Certainteed MemBrain that is taped at seams and sealed around the perimeter of the ceiling.
With all of these approaches the the ceiling air barrier should be continuous with the exterior wall air barrier, with no gaps. It it also important to seal any holes in top plates of walls and partitions that might bypass the ceiling air barrier, allowing a path for moisture into the the roof cavities. Finally, don’t installed recessed lights or other ceiling fixtures unless they are special air-sealed units. Don’t make any other holes in the ceiling air barrier (pic below).
Roof ventilation also helps, but it should not be the main defense against condensation under the sheathing. Its main purpose is to reduce ice dams in winter and reduce overheating of the roof in summer. Roof ventilation may also increase the lifespan of roofing shingles. You should definitely keep the roof ventilation unless you are using a “hot roof” design with closed-cell spray foam – a workable, but expensive approach.
With moderate indoor humidity and a tight ceiling air barrier, you can use whatever type of insulation you want. If you have the headroom to place 1-2 in. foam boards below the joists, that’s a good way to go. If you use 1 in. foil-faced foam (R-6-7) plus a 6-in. R-19 batt, you’re getting close to the R-30 minimum for roof insulation required by many codes. It also greatly reduces thermal bridging though the joists. Any type of foam board will work.
The R-19 batts leave 2 in. for the recommended vent space under the roof sheathing. High-density 6-in. batts provide R-21. If you can add an air space below the foil facing by furring down the drywall, you get an extra R-3 approximately.
There’s nothing magical about the R-30 number, which is built into the codes based on the lower cost of attic insulation vs. wall insulation, but does not account for the higher cost of cathedral ceiling insulation. The higher insulation requirement in roofs has nothing to with the fact that “heat rises” as many people believe. But it is the rule in most energy codes. — Steve Bliss, BuildingAdvisor.com